See also #30404
`PrimaryKey` is used to hold pk values for both int64 & varchar data
type. Since it is an interface it may occupies more memory than pure
slices when holding a group of pks.
This PR add `PrimaryKeys` interface when some other module need to hold
lots of PrimaryKeys.
By using this interface, it could reduce the memory of pk slice to half
when using Int64 Pk data type and reduce interface cost for each row of
varchar as well.
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Signed-off-by: Congqi Xia <congqi.xia@zilliz.com>
